This invention relates to carbon-carbon composite materials, particularly reinforced carbon-carbon composites for reentry vehicle nosetips.
Recent advances in the field of aerospace technology have created a need for high strength, temperature-resistant materials that possess the necessary properties needed to protect reentry vehicles from the severe thermomechanical stresses encountered within the reentry environment. A wide variety of reinforced, three-dimensional composite structures are now available. These composites generally consist of graphite fiber reinforcement which is oriented in at least three directions and a graphite matrix. The simplest of these structures is three-directional (3D) with reinforcing elements which are mutually orthogonal. The most complex ia a thirteen-directional (13D) structure. The 3D structure can be constructed by piercing perpendicular to a stack of parallel woven fabric, or by orthogonal weaving.
The nosetip of a reentry vehicle is required to withstand heating encountered during atmospheric entry, maintain the structural integrity of the vehicle, prevent overheating of the payload, and maintain the aerodynamic characteristics of the vehicle. As the vehicle travels through the Earth's atmosphere it experiences frictional heating in the boundary layer at its surface. The nosetip is also subjected to heat from gases that are at elevated temperatures as a result of being decelerated by the bow shock wave.
The amount of heat transferred to the nosetip depends on its shape and the materials from which it is made. The bow shock wave heats the gases behind it. The heat reaches the nosetip in the form of convection and radiation through the boundary layer adjacent to the surface.
For a slender vehicle with a sharp nose, the bow shock wave is relatively weak and lies fairly close to the body, resulting in a small wave drag. A proportionally small amount of air is heated, and the friction drag is high. For a blunt body, the bow shock is much stronger, extends further away from the vehicle sides, creates larger wave drag, and heats a considerably greater gas volume than its sharp, thin counterpart. The heat applied to the vehicle in the latter case is markedly less than the former case, because a greater fraction is absorbed in heating the atmosphere.
Even for a properly designed shape, it is inevitable that some fraction of the vehicle's initial kinetic energy will finally reach the nosetip in the form of heat. Ablation is used to provide surface protection. Heat can be diverted from the reentry vehicle by allowing the nosetip's outer layer of material to melt, vaporize or sublime. While ablation provides excellent thermal protection, the resulting change in profile due to surface recession can adversely change the aerodynamic characteristics of the vehicle. Additionally, adequate strength must be provided to prevent mechanical erosion of the nosetip by aerodynamic shear stresses.
Although the nosetip is a sacrificial item, it is desired that ablation be controlled, i.e., that the nosetip profile remain substantially the same throughout the period of reentry, under any weather conditions, which may range from essentially clean air to high levels of dust and water droplets.
It is known to incorporate a particulate material into a carbon-carbon reinforcing structure to provide resistance to recession. Such particulate materials include ceramic materials such as silicon dioxide, silicon nitride, silicon carbide, titanium carbide, and the like. Generally, such particulate material is introduced into the reinforcement structure as a dry powder, as a paste or in liquid suspension, in which the particles must be small enough to pass through the interstices of the reinforcing matrix.
Stover, U.S. Pat. No. 4,400,421, discloses that TaC may be formed in situ by impregnating a reinforcement structure with a mixture of tantalum oxalate and a sugar, heating to pyrolyze the organic matter and thereafter heating to about 2700.degree. C. to form TaC.
The problem with the methods for impregnation or loading used heretofore is that such methods either load the reinforcing structure throughout or that only the outer surface of the structure is loaded. Heretofore there has been no viable method for selectively loading a carbon-carbon structure with a desired particulate material.
Accordingly, it is an object of this invention to provide a process for fabricating a carbon-carbon reinforcing structure which is selectively loaded with a desired particulate material.
Another object of this invention is to provide a nosetip for a reentry vehicle which is selectively loaded with a desired particulate material.
Other objects, aspects and advantages of the present invention will be apparent to those skilled in the art from the following description of the invention.